Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 116

DOAS. These measurement techniques allow for temporally continuous obser-
vations. Path-averaged data are obtained for monitoring purposes as well as for
evaluation of numerical model results. For the latter purpose path-averaged data
are much better suited than punctual in-situ measurements, because they are aver-
aged over length scales that are comparable to the grid distances of the numerical
models.
With FTIR CO, CO 2 ,CH 4 ,N 2 O, NO, NO 2 ,SO 2 ,NH 3 ,SF 6 , HCHO, BTX
(benzine, toluene, xylene), as well as n -pentane, ethane, ethene, ethine, propane,
propene, (iso-)butane, iso-butene, butadiene, and hexane can be detected. To inter-
pret infrared spectra of atmospheric measurements, a multi-component air pollution
software (MAPS) was developed for retrieval of gas concentrations from radiation
emission as well as absorption measurements (Schäfer et al. 1995 ). In absorption
mode, a radiation source is needed (either the sun or an artificial heat source). The
upper left frame in Fig. 4.8 shows a methane concentration time series obtained
from FTIR measurements.
FTIR in emission mode does not need a radiation source. Thus imaging by scan-
ning is possible. The scanning infrared gas imaging system (SIGIS-HR), which is
based on the FTIR technology, together with the quantitative gas analysis software
MAPS offers the investigation of the spatial distribution of the temperature and gas
concentrations (e.g. CO, NO) within hot plumes of aircraft engines at airports. The
system integrates an infrared camera as well for the localization of the hot source
that additionally suggests the best measurement position of the SIGIS-HR (Flores
et al. 2007 ).
With DOAS, BTX, and other reactive gases such as O 3 ,NO 2 ,NO 3 , (OH),
HCHO, HONO, and SO 2 can be detected. A comparison of different DOAS instru-
ments is presented in Camy-Peyret ( 1996 ). Recent advances in tropospheric chem-
istry observation by spatially resolving spectroscopic techniques like active and
passive DOAS tomographic measurements of two-dimensional trace gas distribu-
tions, as well as ground-based and airborne Imaging DOAS (I-DOAS) observation
of 2D and 3D trace gas patterns, are described by Platt et al. ( 2009 ). A par-
ticularly promising approach is the combination of tomographic techniques with
imaging - DOAS on airborne platforms, which can provide three-dimensional trace
gas distributions.
An example for the determination of vertical gradients of NO 3 ,N 2 O 5 ,O 3 , and
NO x concentrations in the nocturnal boundary layer with DOAS measurements
along slanted paths is given in Stutz et al. ( 2004 ). Two DOAS instruments were
mounted in 2 m height on an airport and five different retroreflector arrays were
deployed in some distance at different heights, with the goal of measuring the ver-
tical distribution of the trace gases. Three arrays were mounted on a radio tower at
6.1 km distance from the DOAS telescopes at 70 m, 99 m, and 115 m height above
the ground. Another retroreflector array was mounted on a water tower at 1.9 km
distance and an altitude of 44 m. The last and shortest light path ran 2 m above the
meadow inside the airport. Results from this experiment are vertical trace gas pro-
files over the first 115 m above ground with roughly 20 m vertical resolution. These
profiles are horizontally averaged over the DOAS path lengths.